Trialkoxysilanes, especially trimethoxysilane, triethoxysilane and tri(isopropoxy)silane, are used in the production of silane coupling agents. One method of synthesis of trialkoxysilanes is directly from silicon and an alcohol in the presence of copper or a copper compound. This method is known variously in the art as the Direct Synthesis, the Direct Reaction, the Direct Process or the Rochow Reaction. For trialkoxysilanes, it is most conveniently performed in slurry reactors. In a slurry reactor for the Direct Synthesis of trialkoxysilanes, catalytically-activated silicon particles are maintained in suspension in a thermally stable, high boiling solvent and are made to react with an alcohol at an elevated temperature. The trialkoxysilane product is recovered by distillation.
The rate of the Direct Synthesis of a trialkoxysilane is the temporal consumption of the raw materials, i.e., alcohol or silicon, or the temporal formation of products, i.e., trialkoxysilane, and optionally including byproducts. Familiar units are weight percent silicon conversion per hour, or kilograms product per kilogram silicon per hour.
Selectivity is the preference for the trialkoxysilane under the reaction conditions. It is expressed herein as the gravimetric ratio, i.e., trialkoxysilane/tetraalkoxysilane. However, some publications use the molar percentage, 100 (moles trialkoxysilane/molar sum of all silicon-containing products).
Stability of the reaction is the maintenance of desirable rate and selectivity until all raw materials are consumed, or consumed beyond a preset criterion. The progress of the Direct Synthesis can be monitored by determining product composition and/or reaction rate as a function of time or silicon conversion. In general, the profile shows an initial period, termed the induction period, of increasing rate and increasing trialkoxysilane concentration in the reaction mixture, after which the reaction settles into a steady state. In this state, the composition of the reaction mixture remains approximately constant. A period of declining rate and decreasing trialkoxysilane content in the product mixture follows the steady state.
Prior art production of trialkoxysilanes with catalyzed reactions of silicon and alcohols is well known. For example, Rochow, U.S. Pat. No. 3,641,077, discloses a slurry-phase preparation of trialkoxysilanes by directly reacting copper-silicon mass, suspended in silicone oil, with alcohol at 250-300° C. The copper-silicon mass contains about 10 weight percent copper and is prepared by heating copper and silicon above 1000° C. in a furnace in a stream of hydrogen gas. Improved selectivity is displayed in U.S. Pat. No. 3,775,457 to Muraoka et al. wherein polyaromatic hydrocarbon oils are used as solvents in the Direct Synthesis of trialkoxysilanes from an alcohol and finely divided silicon metal activated with cuprous chloride catalyst and HF or HCl.
Suzuki, et al. in Japanese Kokai Tokkyo Koho 55-28929 (1980) acknowledge that activation of silicon with CuCl does not always lead to desirable rate and selectivity. The patent teaches treatment of CuCl with nitrites, naphthalenes, biphenyls and anthracenes prior to its use in the Direct Synthesis. The use of acetonitrile—treated CuCl afforded a rate increase early in the reaction, but this improvement was not sustained at longer reaction times. Selectivity declined considerably relative to the control.
U.S. Pat. No. 4,727,173, to Mendicino, teaches the use of copper (II) hydroxide catalyst to avoid the limitations associated with cuprous chloride and provides a high selectivity to trialkoxysilanes.
U.S. Pat. Nos. 6,580,000 and 6,680,399, disclose the use of copper (II) organophosphate salts and tetraalkyl orthosilicates for the Direct Synthesis of triethoxysilane. However, the processes disclosed therein produce reaction mixtures with selectivity less than one and up to about three.
In U.S. Pat. No. 5,728,858, Lewis, et al disclose that when copper (II) hydroxide is used in combination with alkylated benzene solvents, such as dodecylbenzene, the Direct Synthesis of trialkoxysilanes becomes unstable after approximately 25-35 weight percent of the silicon has been reacted. When methanol is the alcohol reactant at temperatures above about 220° C., trimethoxysilane content in the reaction product declines from approximately 90-95 weight percent to approximately 50-60 weight percent and recovers again to between 80-95 weight percent after about 60 percent silicon conversion. Simultaneous with this loss of selectivity is the enhanced formation of methane, water and dimethyl ether. Methane and dimethyl ether formation represent inefficient use of the methanol reagent. Water reacts with trialkoxysilanes and tetraalkoxysilanes to produce soluble, gelled and/or resinous organic silicates. Formation of these silicates represents inefficiency in the Direct Process. Additionally, the silicates contribute to foaming and incomplete recovery of the reaction solvent as disclosed in U.S. Pat. Nos. 5,783,720 and 6,090,965. U.S. Pat. No. 5,728,858 teaches the reductive activation of copper (II) hydroxide/silicon slurries with hydrogen gas, carbon monoxide, monosilane or polyaromatic hydrocarbons to obtain desirably active, selective and stable Direct Synthesis of trialkoxysilanes in alkylated benzene solvents, such as the linear alkylate, NALKYLENE® 550BL. Reductive activation affords a steady-state region between about 10-70 weight percent silicon conversion, increased silicon conversion and increased selectivity to trimethoxysilane.
The reaction profile described above is generally observed in the Direct Reaction with methanol, but not with the higher alcohols. These alcohols show a maximum in trialkoxysilane formation followed by a decline, with no intervening steady-state period. Some patents, for example, U.S. Pat. Nos. 3,775,457; 5,527,937 and Japanese Kokai Patent Application 6-306083 (1994)) have hinted at this difference with remarks about “waning reactivity” or “maxima in triethoxysilane formation” with time. The rate versus time plots published by Okamoto, et al., J. Catalysis, 143 (1993) pp 64-85 and 147 (1994) pp 15-2, for the Direct Synthesis of trialkoxysilanes in fixed bed reactors show distinct maxima at short reaction times (low silicon conversion) followed by deactivation at longer reaction times (higher silicon conversion) for all temperatures (200-280° C.) investigated. The authors report silicon conversions of 73-92 percent and selectivity 0.26-78 for the Direct Synthesis of triethoxysilane. They also observed this peaked reaction profile for Direct Synthesis of trialkoxysilanes in an autoclave (See J. Organometallic Chem., 489 (1995) C12-C16).
Alcohol dehydration and dehydrogenation are especially troublesome problems when ethanol and other higher homologs are used in the Direct Synthesis. At some temperatures (>250° C.), alkenes, aldehydes and acetals, and not the desired trialkoxysilanes, are formed in significant amounts. Even when these are not the predominant products, their presence in the reaction mixture can result in the inhibition of further catalytic activity. At lower temperatures, (for example 180-220° C.) alcohol decomposition reactions are less prevalent, but the Direct Synthesis is slower.
Co-pending published U.S. patent application Ser. No. 09/974,092, incorporated herein by reference, discloses a process for using nanosized copper and nanosized copper compounds as the sources of catalytic copper for the Direct Synthesis of trialkoxysilanes. Nanosized copper yields high dispersion of catalytic sites on silicon and contributes to improved rate, selectivity and stability at the lower temperatures.
However, in spite of the improvements and advances taught in the cited prior art, there continues to exist the need for a stable, highly selective and rapid Direct Synthesis of trialkoxysilanes, which consumes the raw materials, especially ethanol, propanol and higher alcohols, efficiently, produces less wastes and avoids the deficiencies of unsteady state (peaked profile) operation. In particular, there is a need for such a Direct Synthesis, which exhibits a long period of stable reaction rates and selectivity in its reaction profile. There is also a continuing need for a Direct Synthesis which eliminates or avoids the alcohol reduction, alcohol dehydrogenation and alcohol dehydration side reactions typical of ethanol and the higher alcohols. The present invention provides such a process.